A penetrometer is an apparatus for exploring the ground by driving into the ground a probe located at the lower end of a set of tubes ensuring the transmission of the forces to a measing device placed at the top of this set of tubes.
The conventional measurements which may be carried out by means of a penetrometer, for so-called static operation are measurement of the point force exerted on the probe, measurement of the resistance exerted on the lateral wall of the probe, and measurement of the resistance exerted over the entire length of the set of tubes.
These three measurements take place simultaneously as the set of tubes is driven in. When the set of tubes is extracted only the value of the resistance exerted on the entire length of the set of tubes is measured.
In order to carry out these three measurements easily, the probe and assembly is currently made in three coaxial autonomous parts. The point force is recorded by means of a cap of conical shape, integral with a central rod formed by parts connected one after the other. The resistance exerted on the lateral wall of the probe is transmitted by means of an intermediate tube surrounding the central rod and formed, like the latter, of separate unit members placed end to end. The resistance exerted on the entire length of the set of tubes driven into the ground is applied to an outer tube which serves as a casing for the entire set of tubes and which is also formed by parts connected one after the other.
To measure these various resistances, one normally uses dynamometric rings or casings filled with a hydraulic fluid, in which case the measurement takes place by pressure sensors. These pressure-sensitive means generally lack accuracy and reliability and do not make it possible to record the measurements, considerably slowing down the tests.
A static recording penetrometer has already been described by BENGT B. BROMS and D. E. BROUSSARD in the "Journal of Soil Mechanic and Foundation Division, ASCE" of January 1965 (U.S.A.). The measurement is effected hydraulically and the recording systems generally comprise relatively delicate devices which withstand the hazards of open-air sites quite poorly and require frequent adjustment.
Certain designers have also used electronic extensometry pick-ups of the strain-gauge type, which prove to be very accurate, strong and compatible with various recording methods (see French Pat. Nos. 983,514 and 2,006,848). However, these devices have not been perfected: in particular they facilitate only a very limited number of separate measurements and their pick-ups are housed inside the probe, so that they are connected to the measuring apparatus and/or recording apparatus, located on the outside, by connecting wires which have to be extended as the probe penetrates further.